1. Field of the Invention
This invention relates to a thermal transfer sheet having a hot-melt ink layer. More particularly, this invention relates to a thermal transfer sheet which is suitable for use in printing of an image having stability such as chemical resistance and heat resistance.
2. Background Art
A thermal transfer sheet comprising a substrate and a hot-melt ink layer provided on one side of the substrate has hitherto been used as a thermal transfer recording medium for use with a thermal printer, a facsimile machine, a bar code printer and the like.
In the conventional thermal transfer sheet, about 10 to 20 .mu.m-thick paper, such as a capacitor paper or a paraffin paper, or about 3 to 20 .mu.m-thick plastic film, such as a polyester film or cellophane, is provided as a substrate film, and a hot-melt ink of wax with a colorant, such as a pigment or a dye, incorporated therein is coated on the substrate film to form a hot-melt ink layer.
In use, heat and pressure are applied through the back side of the substrate film by means of a thermal head to melt the hot-melt ink layer in its areas corresponding to printing areas and to transfer the melt onto a printing paper, thereby conducting printing.
In recent years, in the production of members like assemblies of precision components, such as cathode-ray tubes and IC circuits, the above thermal transfer sheet is, in some cases, used in such a manner that bar codes are printed using the thermal transfer sheet on a label and the printed label is applied to members for purposes of process control. These members, in the course of production process, are treated under severe conditions, that is, undergo sintering at a high temperature of about 500.degree. C. or surface treatment with a chemical, such as an acid. In this case, the bar code label is also exposed to the same severe conditions. In some thermal transfer sheets for such an application, a heat-resistant glass frit is dispersed in a binder for the hot-melt ink layer of the thermal transfer sheet (Japanese Patent Laid-Open No. 198195/1984).
In the case of the thermal transfer sheet containing a glass frit in the binder of the hot-melt ink layer, lead glass has been used as the glass frit, posing a problem that lead becomes a toxic water-soluble compound. Therefore, this thermal transfer sheet is unfavorable from the viewpoint of biological safety.
Further, when the conventional thermal transfer sheet is used for the above process control, the binder of the hot-melt ink layer is evaporated from the printed label upon heat treatment, leaving only a pigment. Therefore, the printed image is not fixed on the label, posing a problem that the image has no storage stability.
An object of the present invention is to solve the above problems and to provide a thermal transfer sheet which can provide an image having excellent storage stability and biological safety, that is, creating no toxicity, in applications where sintering at a high temperature and surface treatment with a chemical are conducted.